Purification of perfluoroheptane by azeotropic distillation with methyl ethyl ketone



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June 1, 1948.

Patented '.June l, i948 PURQIFICATHON F PERFLUOBOHEPTANE BY AZEOTBOCDISgILLATION WITH METHYL ET it KET Latimer B. Evans, Wilmington, Del.,and Henry B. Hass, West Lafayette, Ind., mignon to the United States ofAmerica as represented by the -lnited States Atomic Energy CommissionApplication March 19, 1948, Serial No. 655,584

d Claims.

This invention relates to the purification of perfiuoroheptane. Itparticularly relates to the separation of chloroiluorocarbons frommixtures of periluoroheptane with such compounds.

Peruoroheptane may be prepare by the reaction of metal peruorides, suchas cobalt trifluoride, manganese-triuoride and silver difluoride, withnormal heptane. However the manufacture of the metal perfiuoridesemployed in such processes is a relatively costly procedure because itrequires the use of elemental uorine. In order to minimize thequantities of such reagents employed in the manufactureofperiluoroheptane, a process has been developed involving chlorination ofnormal heptane to an average chlorine content of ten to twelve chlorineatoms per molecule, iluorination oi the resulting polychloroheptanemixture by treatment with hydrogen fluoride in the presence ofpentavalent antimony iiuorochlorides, and treatment oi' the more highlyfluorinated fractions of the resulting product with cobalt trifluorideor silver diiiuorlde. This latter procedure results in the consumptionof substantially smaller quantities of the uorldes requiring elementaliluorine in their preparation but has the disadvantage that since someof the chlorinecontaining organic icy-products boil in the same range asperfluoroheptane and it has been im possible, as a practical matter, toeliminate these .impurities by simple rectification, the product asobtained in the iinal treatment normally includes between 2% and 7% oforganic chlorine.

It is an object of the present invention to purify periiuoroheptane. Itis a further object of the invention to remove associated impuritiesboiling from the crude product in the same temperature range as theperiluoroheptane. A further object of the invention is the production ofperfiuoroheptane free from chlorofluorocarbon by-products. A stillfurther object is the accomplishment,

of these purposes in a simple and eilicient manner without resort tocomplex chemical operations. Further objects will appear from thefollowing general description of the invention and detailed examples ofits application.

In accordance with the present invention a crude peruoroheptane mixturecontaining chlorofiuorocarbons boiling from the mixture in the sametemperature range as the periiuoroheptane is subjected to fractionaldistillation with methyl ethyl ketone at a vapor temperature below thetemperature of distillation of the chlorouoro cai-bons from the mixture.

The crude peruoroheptane fraction obtained by fractional distillation ofthe product of cobalt triiiuoride or silver diiiuoride treatment of themore highly fluorinated compounds obtained in the chlorination, hydrogenfluoride uorination process boils between about 80 C. and about 85 C. atnormal atmospheric pressure. Methyl ethyl ketone boils at about 79.6; Onthe other hand, a mixture oi' the perfluoroheptane fraction with asubstantial proportion of methyl ethyl ketone has an initial boilingtemperature of about 62-63 C. By fractionally distilllng the mixture soas to retain higher-boiling fractions in the distill and. a distillatefraction essentially free of chlorine is obtained. Part o! the methylethyl ketone may be separated fromthe perfluoroheptane by decantationand the dissolved ketone may be removed by washing with water and, ifdesired, by washing` with aqueous sodium bisulite, NaHSOn, solution.Since peruoroheptane is substantially insoluble in water whereas methylethyl ketone is soluble therein, satisfactory separation of theperuoroheptane and methyl ethyl ketone can be eected by water-washingalone. Methyl ethyl ketone is soluble in pure peruoroheptane only totheextent of about 10% by weight; hence most of the ketone may be separatedfrom the perfiuoroheptane distillate by decantation. In order Ato stillfurther reduce the quantity of methyl ethyl ketone to be removed bywater-washing, the purified perfluoroheptane, after decantation of theketone phase, may be redistilled to remove a ketone azeotrope, leavingas `distillation residue periluoroheptane free from methyl ethyl ketone.The distillate of this distillation then maybe subjected to decantationto separate the ketone from the perfluoroheptane, and the latter,containing dissolved ketone, may be either washed with water or returnedto the still for distillation with a subsequent batch ofketone-eontamlnated 'perfiuoroheptane Since one of thechlorofluorocarbon impurites normally associated with perfluoroheptaneobtained by the process described forms with methyl ethyl ketone anazeotrope distilling, at normal atmospheric pressure, at about 65.5 toabout 66.8 C., the distillation should be controlled to avoid adistillation temperature rise of more than about 2 C. above thedistillation temperature of the perfluoroheptane methyl ethyl ketoneazeotrope.

A preferred method of purifying peruoroheptane as discussed in the priorparagraphs is illustrated in the accompanying diagrammatic drawing. Inthis preferred process a i'eed mixture oi crude periiuoroheptane andmethyl ethyl ketone is distilled in a fractionating column and thevaporized azeotropic mixture of periluoroheptan and methyl ethyl ketoneis condensed in a conde er and then fed into a decanter where most ofthe methyl ethyl ketone is separated from the peruoroheptane bydecantation. The remainder of the methyl ethyl ketone is removed byredistilling in a fractlonating column the periluoroheptane containingdissolved methyl ethyl ketone and condensing the vaporized ketoneazeotrope in a condenser, thus leaving a distillation residue ofperiluoroheptane free from methyl ethyl ketone. The condensed vaporizedketone azeotrope of this distillation is then fed into a decanter, andmost of the methyl ethyl ketone is separated from the perfluoroheptaneof the azeotropic mixture by decantation. The latter periluoroheptanecontaining dissolved methyl ethyl ketone is then mixed with water andfed into another decanter where .the methyl ethyl ketone is separatedwith the water from the periiuoroheptane by decantation.

While the process is capable of satisfactory operation at pressureseither above or below atmospheric pressure, provided lthe distillationtemperatures are adjusted to compensate-for the variation in pressure,standard atmospheric pressure distillation is entirely eifective andtherefore resort to either of these alternatives is not necessary.

The codistillation of the crude peruoroheptane with methyl ethyl ketonemay be carried out in conventional fractional distillation equipment ofeither the batch type or continuous type.

The proportion of methyl ethyl ketone required for eifecting thedistillation of a pure periiuoroheptane from crude mixtures thereof isnot critlcal. A small proportion of the periluoroheptane in pure formmay be recovered using very low ratios of methyl ethyl ketone to thecrude material. However, in order to recover all of the peruoroheptanefrom a crude mixture it is necessary to supply sumcient ketone toprovide a ketone to perfluoroheptane ratio in the distillate vaporscorresponding tothe ratio in the azeotrope (about 4:7). The quantity ofmethyl ethyl ketone necessary for accomplishing this result may bepresent at the beginning of the distillation or it may be added asthedistillation proceeds. In

4continuous'operations it is expeditious to add the crude material andthe methyl ethyl ketone at an intermediate point in the distillationcolumn.

The following examples further illustrate the process of the invention.

Empr-.r 1

separating from the resultant product a iraction distlliing below 138C.. (4) treating this fraction with cobalt triuoride at a tempera tureof 300 C. to 350 C. and (5) separating from the resultant iluorination.Iproduct a distillate fraction boiling at 80-f85 C. at atmosphericpressure. Seventy-nine miililiters of the crude 30-85 C. fractioncontaining 2.9% organic chlorine was washed by mixing it with an equalvolume of methyl ethyl ketone and separating the ketone phase from theperuoroheptane phase. The washed perlluoroheptane, (77 mi.) whichcontained 2.6% chlorine and a minor proportion of dissolved ketone, wasthen mixed with an equal volume of fresh methyl ethyl ketone and themixture was fractionally distilled in a 4 ft. Podbielniak Hyper-calcolumn.

'Ihe distillate fractions were washed with wa- Crude periluoroheptanewas obtained by a process which involved (1) chlorinating normal heptanein the presence of light using gaseous chloproduct with hydrogeniluoride in the presence of fluorochlorides of pentavalent antimon'y,(3)

and chlorine contents of the washed fractions were determined. The dataare tabulated below.

Table I Volume Distlilation Total Refractive l' rac- After Por contTompora- Volume, Index at on turc, C. mi. Wflmg m C. cl

A... i530-81.2 3.2 1.3 1.270 0.8 B. 61.2-63-0 50.6 46.2 1.265 0.263.0-65.7 6.7 4.2 1.276 4.9 D 65.74310 6.6 4.4 1.284 9.8 E 67.072.6 8.13.2 1.5111 13.2 Res 60.5

The molecular weight of washed fraction B was 386 confirming theidentity of this fraction as essentially pure periluoroheptane.Moiecular weights of 351 and 313 for washed fractions D and E indicatedthat these were probably six-carbon perhalides, or in the case offraction E, possibly five-carbon perhalides, and their chlorine contentindicated that they contained one chlorine atom per molecule.

Exlunu: 2

Two hundred fifty-two milliiiters of crude periiuoroheptane product(which was obtained as described in Example 1: had a boiling range of80- 84 C.: contained 4.0% chlorine. 75.8% uorine, and 0.077% hydrogen;and had an average molecuiar weight of about 366) was mixed with 300milliiiters of methyl ethyl ketone and the mixture was rectied in thesame equipment employed in Example 1. Several fractions were collectedand in each case the total distillate was rst measured: then the ketonephase was decanted oi! from the fluorocarbon phase and the latter waswashed with water to remove ldissolved ketone. The volume of the washediiuorocarbon phase was then measured and data as to chlorine content,iluorine content, molecular weight, and refractive index determined.'I'he data obtained in th-is manner are tabulated below:

Table Il Dlstiilation T Refractive otal Vol- Volume After s PercentPercent Molecular "59mm Tm?' e um, mi. Washing, ml. Indeff' 2 c F Weight40-61. 4 l5. 8 8. 8 1.272 l. 2 75. 7 321 6l. 4-62. 2 1 13.3 7.0 l. 265(l. 7 77:5 W 62.263.3 284.5 160.0 LM 0.0 78.7 381 L3-66.5 4.8 2.7 1.27365.566.8 64.8 41.2 1.287 0.4 70.0 335 66.8-7L7 5.3 2.2 1.306 71. 'I-72.3 18.7 8.1 1.324 22. 5 57. 4 $8 72. 343.6 1. 331 22. 6 55. l 277 Forfurther puriilcation fraction C was washed with aqueous NaHSOs solutionand then subjected to distillation with rectication without the additionof an azeotropic agent. A foreshot distilling from 79.0 to 81.6 C. wasthus obtained. The remainder of the fraction distilled at 81.6- 81.9 C.This distillate was found to contain 0.0% Cl, 0.01% H, and 77.9% F andto have a molecular Weight of 387-388.

It will be understood that we intend to include variations andmodifications of the invention and that the preceding examples areillustrations only and in no wise to be construed as limitations uponthe invention, the scope of which is deilned in the appended claims,wherein We claim:

.1. The method of purifying perfiuoroheptane contaminated withimpurities resulting from the method of its preparation by uorinatingpolychlorinated heptane and boiling from the crude mixture in the sametemperature range as the perfiuoroheptane,1 which comprises mixing thecrude perfluoroheptane with methyl ethyl ketone and fractionallydistilling the mixture at a vapor temperature below the distillationtemperature of said impurities from the mixture.

2. The method of purifying peruoroheptane containing chlorouorocarbonsboiling from the crude mixture in the same temperature range as theperfluoroheptane, which comprises. mixing the 30 2,352,268

3. The method of purifying a crude periluoroas heptane obtained by thenuorinatlon of 'a polythe perfluoroheptane distillate product is furtherpurified by a fractional distillation to remove dissolved methyl ethylketone; leaving as distillation residue puriiled peru'oroheptanesubstantially free from innate chlorofiuorocarbon impurities and methylethyl ketone.

LATIMER R. EVANS.

wmf. l'. B. HASS.

REFERENCES CITED The following references are of record in the ille ofthis patent:

UNITED STATES PATENTS Number Name Date 2,341,433 Fisher Feb. 8,1944Kimberlin June 27, 1944 2,388,040 Clark Oct. 30,19'45 FOREIGN PATENTSNumber Country Date 616.596 Germany `Aug.'l, V1935

